Followup to getodk/central-backend#1628

Consider rejecting if there are multiple datasets declared in a form but the entities-version spec is older than 2025.1.

Discuss further: Forms with entities from repeats will most likely be created with pyxform, which will automatically set the entities-version spec to the right version of 2025.1.0. Is this check necessary?

In the central backend code, the entities version string is validated at the time of parsing the form XML for dataset information. It is not used outside of this code. At that point, there isn't enough information to know if an entity declaration is with a repeat, because that happens in a later step when the dataset declarations are merged with the form fields. We no longer have access to the entities version string then. We could pull it out and save it to inspect later... or we could not do this check.